How OBC’s PI Control Manages Power Delivery to Aux Loads Without Charging the EV Battery

How OBC’s PI Control Manages Power Delivery to Aux Loads Without Charging the EV Battery

Read Current Control in AC Charging for EVs and Current Control in DC Charging for EVs to understand this article better.

This article focuses on AC charging with an Onboard Charger (OBC), but the same principles apply to DC charging as well. For more details, refer to the articles on AC and DC charging above.

In electric vehicle (EV) charging systems, there are instances where the onboard charger (OBC) must supply power solely to the vehicle’s auxiliary loads—without delivering any current to the high-voltage (HV) battery. This typically occurs when the battery is fully charged, but the vehicle still requires power for auxiliary systems such as cabin air conditioning, battery thermal management, or onboard electronics.

A common technical question in this context is:
How does the OBC’s control ensure that power is delivered only to auxiliary loads, while preventing any current flow into the battery?

This article explains the core working principle behind this functionality and how PI control dynamically regulates the OBC output voltage to maintain zero battery current while still powering auxiliary systems.

High-Level System Overview

At steady-state, the high-level circuit in this scenario includes:

  • OBC supplying a regulated DC output voltage (V_obc).
  • HV battery connected to the same DC link as the OBC.
  • Auxiliary loads (e.g., HVAC, cooling systems, and onboard electronics) drawing power from the DC link.
Higl level representation of OBC charging the battery

Let’s consider the case where the OBC output voltage (V_obc) and the battery terminal voltage (V_bat) are both at 400V. A common assumption might be that, since these voltages are equal, both the OBC and the battery will share the load current. However, this assumption does not hold true in real operation.

The Role of Battery Internal Resistance

To understand why the battery doesn’t supply or absorb current in this condition, we need to factor in the internal resistance (R_i) of the battery.

Higl level representation of OBC charging the battery with internal resistance

When there is no current flowing into or out of the battery, the terminal voltage (V_bat, which includes the voltage drop across R_i) equals the battery’s open-circuit voltage (OCV). This situation represents an electrical equilibrium where the battery neither charges nor discharges.

Key Point: If the OBC maintains an output voltage equal to the battery’s OCV, and no net voltage difference exists across R_i, then the current (I_bat) remains zero.

This is the basis on which the PI controller operates to isolate the battery during auxiliary load operation.

How PI Control Maintains Zero Battery Current

The PI controller in the OBC plays a vital role in dynamically adjusting the output voltage (V_obc) to ensure zero net current into or out of the battery.

1. OBC Output Voltage Regulation

  • The PI controller continuously receives real-time feedback of the battery current (I_bat).
  • Based on this feedback, it fine-tunes the OBC output voltage to a point where the battery current is exactly 0A.
  • When this balance is achieved, V_OBC matches the battery’s OCV, and the battery remains electrically isolated from power flow.

2. Steady-State Operation

  • Once the PI controller stabilizes, all auxiliary loads are powered exclusively by the OBC.
  • The battery remains unaffected, with no charging or discharging activity occurring.
  • This allows critical systems to operate even when the battery is fully charged—without violating charging constraints or risking overcharging.

Dynamic Response to Load Variations

Auxiliary loads in EVs are not constant—they can change abruptly due to system demands. Here’s how the PI-controlled system responds in real time:

a. Sudden Decrease in Auxiliary Load

  • Initially, the OBC may continue supplying higher current, resulting in temporary charging of the battery.
  • The PI controller quickly detects this and regulates the OBC voltage, pulling the battery current back to zero.

b. Sudden Increase in Auxiliary Load

  • If the load demand rises suddenly, the battery might momentarily discharge to support the load.
  • The PI controller detects the deviation and responds by regulating the OBC current, restoring the battery current to zero again.

This closed-loop regulation ensures the battery remains protected and uninvolved, even during transient conditions.

Practical Considerations

1. Accuracy of Battery Current Measurement

  • Accurate battery current sensing is essential for effective PI control.
  • Any measurement inaccuracies can cause unintentional charging or discharging of the battery.

2. Speed of OBC Voltage Regulation

  • The response time of the PI controller plays a critical role in maintaining system stability.
  • Tuning the PI control parameters—proportional gain (K_p) and integral gain (K_i)—is essential to achieve fast and stable voltage adjustments without introducing oscillations or overshoot.

The PI controller in an onboard charger (OBC) serves a crucial role in managing power flow during EV operation. By dynamically regulating the output voltage, the PI control system ensures that auxiliary loads receive the necessary power without any current entering or exiting the battery.

Amar Reddy Avatar
Amar Reddy, a seasoned professional in EV Charging with more than a decade of expertise in the Electric Vehicles domain. With a Master’s degree in Electrical Engineering, he is eager to share valuable insights into EV charging, catering to enthusiasts and fostering a deeper understanding of the field.

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